If you get speakers will you be powering them by an aftermarket amp? or by the HU?

I would definitely get speakers as long as you are powering them by an amp. All the sub/amp will do is fill in the lower end. No point in doing that yet if you can't hear the actual music. The factory speakers are absolutely terrible, don't let anyone fool you.

I would suggest calling Mr. Marv. I was pointed in his direction after reading a few posts. He is renown for his knowledge. He will point you in the right direction. He will most likely tell you to pick up Hybrid Audio Comps and a 4-ch amp lol.... But boy is it awesome... Ask me how I know

The biggest difference you will get in the vehicle is properly treating the doors. See here for an example. This will greatly improve the sound, even the factory speakers.

Next, *with properly treated doors* a speaker replacement will make a huge difference, and if you are not a bass head, you may not feel the need for a sub. But if you decide to go this route, I would probably amp the door speakers.